Authors: Li Shi Harry H Hendon Oscar Alves Matthew C Wheeler David Anderson Guomin Wang
Publish Date: 2010/10/28
Volume: 37, Issue: 1-2, Pages: 313-324
Abstract
The importance of initializing atmospheric intraseasonal stochastic variations for prediction of the onset of the 1997/1998 El Niño is examined using the Australian Bureau of Meteorology coupled seasonal forecast model A suite of 9month forecasts was initialized on the 1st December 1996 Observed ocean initial conditions were used together with five different atmospheric initial conditions that sample a range of possible initial states of intraseasonal stochastic variability especially the MaddenJulian Oscillation MJO which is considered the primary stochastic variability of relevance to El Niño evolution The atmospheric initial states were generated from a suite of atmosphereonly integrations forced by observed sea surface temperatures SST To the extent that low frequency variability of the tropical atmosphere is forced by slow variations in SST these atmospheric states should all represent realistic low frequency atmospheric variability that was present in December 1996 However to the extent that intraseasonal variability is not constrained by SST they should capture a range of intraseasonal states especially variations in the activity phase and amplitude of the MJO For each of these five states a 20member ensemble of coupled model forecasts was generated by the addition of small random perturbations to the SST field at the initial time The ensemble mean from all five sets of forecasts resulted in El Niño but three of the sets produced substantially greater warming by months 4–5 in the NINO34 region compared to the other two The warmer group stemmed from stronger intraseasonal westerly wind anomalies associated with the MJO that propagated eastward into the central Pacific during the first 1–2 months of the forecast These were largely absent in the colder group the weakest of the colder group developed strong easterly wind anomalies relative to the grand ensemble mean that propagated into the central Pacific early in the forecast thereby generating significantly weaker downwelling Kelvin waves in comparison to the warmer group The strong reduction in downwelling Kelvin waves in the weakest case acted to limit the warming in the eastern Pacific resulting in a “Modoki” type El Niño that is more focused in the central Pacific Our results suggest that the intraseasonal stochastic component of the atmospheric initial condition has an important and potentially predictable impact on the forecasts of the initial warming and flavour of the 1997/1998 El Niño However to the extent that atmospheric intraseasonal variability is not predictable beyond a month or two these results imply a limit to the accuracy with which the strength and perhaps the spatial distribution of an El Niño can ultimately be predicted These results do not preclude a predictable role of the MJO and other intraseasonal stochastic variability at longer lead times if some aspects of the stochastic variability are preconditioned by the evolving state of El Niño or other low frequency boundary forcingThe authors would like to acknowledge Dr Guo Liu for providing the AMIPrun data and helpful comments from Dr Andrew Marshall and Dr Debra Hudson on an earlier version of this paper The comments of two anonymous reviewers are also gratefully appreciated
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